The hollow cathode effect and related high-density hollow cathode plasma was invented in 1916 by F. Paschen in his spectroscopy study of the plasma emission. Hollow forms of his electrodes (short rectangular tubes) for igniting the plasma led to considerably more light intensity than in simple planar electrodes at the same direct current (dc) power. Later studies showed that the principle of this intense discharge is based on geometry of the hollow electrode, where electrons emitted from one cathode wall interact with an equivalent electric field with opposite orientation at the opposite inner wall. Depending on gas pressure and distance between electrode walls the electrons can oscillate between inner walls and enhance substantially the ionization of the present gas or vapor. Such ionization based on pendulum motion of electrons is recognized in the literature as “hollow cathode effect” (P. F. Little et al. 1954). The hollow cathode effect can work also in hollow electrode powered by an alternating current (ac). Typical frequency of ac generators for such purpose is between 105 s−1 and 108 s−1, i.e. including the radio frequency (rf) range. The first rf hollow cathode was described in U.S. Pat. No. 4,521,286 to C. M. Horwitz. It was also found that the anode in the rf hollow cathodes is the plasma itself (a “virtual anode”) in contact with the counter electrode (L. Bárdos et al. 1988). The hollow cathode effect can be generated also by pulsed dc power.
The hollow cathode effect is not generated in all negatively biased hollow electrodes. A hollow electrode can differ substantially from real hollow cathodes unless its geometry is optimized to enhance the gas ionization inside its hollow part by 1-3 orders of magnitude due to the hollow cathode effect. For example, a large-diameter cylindrical electrode where the space-charge sheath thickness is much smaller than the electrode diameter cannot serve as a hollow cathode. Even at lower gas pressures when the sheath is wider, the effect may not take place due to a low number of ionizing collisions. In order to excite this effect at higher pressures, the distance between walls must be reduced due to short mean free paths of electrons. The distance d between opposite walls of the hollow cathode must be at least twice the thickness of the space charge sheath, which depends on the gas pressure p, but also on the frequency and power of the generator used. Moreover, the gas pressure p inside the hollow cathode is typically higher than outside the hollow cathode due to higher temperature caused by the high density plasma inside the hollow cathode and due to a pressure gradient formed in the flowing gas or by evaporated cathode material. Also, presence of magnetic fields can affect the confinement and properties of the hollow cathode plasma. Therefore, different published empirical formulas for estimations of the optimal product p·d for the hollow cathode effect in in dc hollow cathodes are generally not very useful.
A number of patents and publications describe so-called “hollow cathode magnetron sputtering” in systems having targets with hollow geometry, mostly cylinder. The target is always negative in order to attract ions for bombarding and sputtering, hence the term “hollow cathode target” and “hollow cathode magnetron”. However, without the hollow cathode effect, e.g. in large diameter targets or in magnetic fields parallel with the walls of the target where electrons are deflected from their oscillations, such systems are not real hollow cathodes. For example, in U.S. Pat. No. 4,966,677 to H. Aichert et al. a magnetron sputtering apparatus has a hollow cathode target with a cathode base in which a hollow target with cylindrical sputtering surface and cylindrical outer surface is disposed. Neither the parallel magnetic field with the target nor the target geometry allows for the hollow cathode effect. Similarly, in U.S. Pat. No. 5,437,778 to V. L. Hedgcoth, a magnetron sputtering system comprises a hollow longitudinal cathode that is either made from or has its interior wall coated with a material to be sputtered. No hollow cathode effect can be excited in such systems. Similarly, in U.S. Pat. No. 6,283,357 to S. Kulkarni et al. a plate of sputter target material is bonded to a sheet of cladding material and then formed into a “hollow cathode” magnetron target. In U.S. Pat. No. 6,887,356 to R. B. Ford et al. a sputtering target is claimed preferably exhibiting uniform grain structure and texture at least on the sidewalls thereof, but no hollow cathode effect is utilized. Another example is PCT Publication WO 2007/130903 to J. K. Kardokus et al., where methods of forming “hollow cathode” magnetron sputtering targets are claimed. In these targets, a metallic material is processed to produce an average grain size of less than or equal to about 30 microns. Such sputtering target preferably exhibits substantially uniform sputtering erosion, but no hollow cathode effect is utilized. Basic principles of these so-called “hollow cathode magnetrons” are explained e.g. by D. A. Glocker (SVC Technical Conference Proceedings 1995).
Since its discovery in 1974 in U.S. Pat. No. 4,166,018 to J. S. Chapin (later described by P. S. McLeod et al. 1977 and R. K. Waits 1978), the magnetron sputtering device underwent a number of improvements aimed mainly to (i) an increase of the target utilization, (ii) a possibility to sputter magnetic targets, (iii) an increase of an ion flux to the substrate, (iv) an increase of the sputtering rate, (v) an increase of the ionization level in the whole magnetron plasma. Substantial progress in tasks (i) and (ii) has been obtained by optimizing geometry and induction of the magnetic tunnels for plasma confinement, particularly by strong permanent magnets, but also by using hollow targets as explained above. Such efforts also included different systems with moving magnets and culminated in an arrangement for target utilization based on a rotating cylindrical target (U.S. Pat. No. 4,356,073 to H. E. McKelvey, filed 1981, and a number of later patents e.g., U.S. Patent Publication 2009/0260983 to M. A. Bernick, filed in 2009). A considerable advance in task (iii) was obtained by partial opening of the magnetic tunnel from so-called “closed field magnetrons” to so-called “unbalanced magnetrons” (B. Window et al. 1986). This solution allows for an enhanced presence of ions (plasma) close to the substrate, efficient biasing of the substrates and controlled growth of e.g. very hard films and special film textures. It is noteworthy that so far the progress in task (iv), i.e. in increasing the sputtering rates, has been obtained by increasing target erosion areas, while different arc evaporators rather than any sputtering devices continued to be relied upon as the fastest physical vapor deposition (PVD) systems. An increase in the ionization of the magnetron plasma (task (v)) can obviously increase the sputtering rate, such as, for example through additional ionization by an rf coil (S. M. Rossnagel et al., 1993). However, recent trends are focused rather to high power impulse magnetron sputtering (HiPIMS) systems disclosed in U.S. Pat. No. 6,296,742 to Kouznetsov (filed in 1997), where the high power peaks increase the ionization dramatically. However due to the pulsed power regime, the average coating rate barely reaches rates comparable to conventional dc magnetrons. Thus, the advantage of favorable coating properties available in high-density plasma of HiPIMS is outweighed by the necessity of complicated and expensive pulsed power generators and so far also by an unimpressive deposition rates.
A direct method of increasing the sputtering rate by increasing the ionization in the magnetron plasma using the hollow cathode has been patented by J. J. Cuomo et al. in May 1986 in U.S. Pat. No. 4,588,490 “Hollow cathode enhanced magnetron sputtering device”. Cuomo et al. combine a hollow cathode as an electron-emitting device with a plasma sputter etching/deposition device such as a magnetron. The hollow cathode is utilized to provide additional ionization of the working gas during magnetron operation and can provide main ionization of the working gas at low magnetron powers. The hollow cathode utilizes thermionic electron emission to inject electrons. For this purpose, it comprises a hollow tubular member constructed of a refractory metal and a plurality of layers of electron emissive foils. The hollow cathode is powered by a dc power source independent on the magnetron power generator. In the preferred configuration, the axis of the cylindrical hollow cathode is parallel with the planar magnetron target and positioned above the target close to its edge. In order not to impede the magnetron drift current the radial position of the hollow cathode must be such that the magnetic field lines that it intersects travel to the center pole, rather than the bottom of the magnetic assembly. Thus the patent discloses an application of a thermionic hollow cathode emitting electrons, without electrical or physical impediment of the magnetron drift current, but also without magnetic enhancement of the hollow cathode plasma. The cathode should be fabricated from refractory metals (e.g. Ta). Low-pressure thermionic regimes of the hollow cathode allow for about 10-times lowering of the magnetron operation pressure, i.e. down to 4-6.7×10−2 Pa (0.3-0.5 mTorr). This type of use of the hot hollow cathode arcs as an auxiliary ionizer in magnetrons is described in the literature as “magnetrons with additional gas ionization” (J. Musil et al., 2006, p. 71-72). An important requirement in such processes is that the cathode metal should not be released and mixed with the sputtered material from the magnetron target.
Another way for involving the hollow cathode plasma in the magnetron discharge is formation of grooves or bores directly in the target in order to excite the hollow cathode effect inside these grooves or bores (J. Musil et al., 2006, pp. 91-93). Such arrangements decrease the necessary voltage for the magnetron discharge, but the sputtering rate decreases as well. Moreover, the targets have rather complicated forms and as the target is consumed during magnetron operation the depths of such hollow cathodes are reduced, requiring changes in power parameters.
In addition to their ability to generate very high-density plasmas (comparable to HiPIMS), due to the hollow cathode effect, hollow cathodes can be used for both ion sputtering and arc evaporation where the cathode itself is a PVD target. Besides dc power, the hollow cathodes can be advantageously powered by pulsed dc or ac electric power (up to the rf range), and can be used to activate gases for fast plasma-enhanced chemical vapor deposition (PE CVD) regimes. Shapes and dimensions of the cathodes can be designed for a wide range of working gas pressures from about 1.33×10−2 Pa (10−4 Torr) up to atmospheric and higher pressures. Besides conventional tube shaped cathodes, the pendulum motion of electrons can also occur between parallel conductive “plates” (with rf power even when plates are coated by dielectrics) to produce dense hollow cathode plasma. Moreover, the hollow cathode effect can be enhanced and focused in selected areas (hot zones) by suitable magnetic fields, as disclosed in U.S. Pat. No. 5,908,602 to L. Bárdos et al. (1994). Main part of the magnetic induction lines (induction vector B) in the slit between parallel linear plates of the hollow cathodes should be perpendicular to the cathode plates in order not to deflect electrons and prevent their oscillations between opposite walls. Electrons moving along vector B are not affected by the magnetic force. However, because the vector E of the electric field in the power circuit is oriented towards the anode, i.e. out of the slit, a considerable disadvantage of the static magnetic field is the tendency to force the plasma to one side of the slit depending on the orientation of magnetic induction vector B. The drift velocity of electrons is given be the vector product (E×B)/B2 (E is the vector of electric field perpendicular to a magnetic induction vector B). The drift velocity vector is perpendicular to both vectors E and B. This insufficiency can be compensated in apparatuses having rotating magnets, as disclosed in U.S. Pat. No. 6,351,075 to H. Baránková et al., where magnetic induction vector B across the hollow cathode slit is changed in both its orientation and amplitude. An obvious disadvantage of such apparatuses is the necessity of mechanical means for driving the magnets.